service: Move client registration into base class

This CL introduces InterfaceWithClientsBase, which is an abstract class
that implements common client registration logic, which will be reused
for Binder interfaces such as IBluetoothGattClient and
IBluetoothGattServer. This also introduces BluetoothClientInstance, so
that per-application API client instances (and their factories) conform
to a common interface.

Bug: 24245347
Change-Id: Ia4b464fa85f7a33c631bf63342631fce8d155b4a